The present invention relates to a liquid tank and, in particular, to a fuel tank with which a motor vehicle may be equipped. It also relates to a method for manufacturing this tank.
Liquid tanks, particularly fuel tanks for motor vehicles, are currently fitted, amongst other things, with a breather circuit. This circuit allows air to be introduced into the tank in the event of depression (in particular to compensate for the volume of liquid consumed) or gases contained in the tank to be removed in the event of overpressure (particularly if the contents warm up). This circuit also allows the gases that are to be discharged into the atmosphere to be routed and possibly filtered with a view to meeting the increasingly strict environmental requirements in this area.
The breather circuit comprises, in the known way, at least one value which as far as possible prevents liquid from the tank from leaving the tank if the tank is excessively inclined or rolls over. This breather valve needs to offer a rapid and reliable response when its operating conditions occur, but have minimal sensitivity to transient phenomena such as, in particular, very high flow rates, overpressures in the tank or low-amplitude waves. It also needs to ensure that a minimum amount of liquid is carried over into the canister (or the chamber containing a substance that adsorbs fuel vapours, usually active charcoal) in normal operation and during filling, otherwise the said canister would become saturated and the removal of contaminants from the gases discharged into the atmosphere would be ineffective. This phenomenon is generally termed LCO (for Liquid Carry-Over) in the jargon of the trade.
Many breather valves employ a float comprising an upper needle valve or ridge that closes off a connecting orifice between the tank and the breather circuit.
The disadvantage with this type of valve is its size and, in particular, its height, which limits the useful volume of the tank when positioned inside the latter. The problem is that the said valve lies at least partially above the maximum level for liquid in the tank, so that it can perform its function. However, it must be noted that a minimum head space known as an “expansion space” is nevertheless required by manufacturers and that, in general, the size and geometry of the valves are tailored accordingly.
Now, at the present time, motor manufacturers are seeking to increase accommodation, modularity and aerodynamics offered by the vehicles. A slim tank is an attractive alternative that allows the total height of the vehicle to be minimized while at the same time maximizing the head room inside the cabin and fitting perfectly under a flat floor.
However, reducing the height of the tank has the effect of limiting the size of the components inserted in it, particularly the ventilation valves because, in proportion, they limit the useful volume far more than they would in normal tanks. In reality, even the smallest float valves available on the market still take up too much of the useful height of slim tanks. In other words: the lost volume is far greater than the expansion volume required by motor manufacturers.
In order to solve this problem, recourse may be had to an expansion volume or chamber external to the tank, particularly as described in U.S. Pat. No. 3,915,184. However, such an architecture entails a separate part and lines of connection between the tank and this part, and this is complicated and expensive.